A conventional traction elevator typically comprises a car, a counterweight and traction means such as a rope, cable or belt interconnecting the car and the counterweight. The traction means passes around and engages with a traction sheave which is driven by a motor. The motor and the traction sheave rotate concurrently to drive the traction means, and thereby the interconnected car and counterweight, along an elevator hoistway. At least one brake is employed in association with the motor or the traction sheave to stop the elevator and to keep the elevator stationary within the hoistway. A controller supervises movement of the elevator in response to travel requests or calls input by passengers.
The brakes must satisfy strict regulations. For example, both European Standard EN 81-1:1998 and the ASME A17.1-2000 code in the United States state that the elevator brake must be capable of stopping the motor when the elevator car is travelling downward at rated speed and with the rated load plus 25%.
Furthermore, the elevator brake is typically installed in two sets so that if one of the brake sets is in anyway faulty, the other brake set still develops sufficient braking force to slow down an elevator car travelling at rated speed and with rated load.
Conventionally, the elevator brakes engage with a rotating component of the motor such as a brake drum or brake discs mounted for concurrent rotation of the motor shaft. Each brake normally has a brake pad that is spring biased towards the surface of the brake drum or disc. Additionally, an electromagnet may be arranged within the brake so that when the coil of the electromagnet is energized it exerts a force on the brake pad to counteract the spring bias and release or disengage the brake pad from the brake drum or disc.
Accordingly, the brake is released or disengaged by supplying electricity to the brake coil through a power supply circuit. Conversely, the brake is engaged by disconnecting the power supply circuit from the brake coil for example with a relay or contactor arranged within the circuit.
Due to the strict safety regulations summarized above, the force exerted by a brake on the drum or disc is substantial, so that when a complete power failure or a disruption such as under-voltage occurs with the commercial mains power supply, the brake pad immediately engages to brake the movement of the elevator car with a large force. The same effect will also be produced if a passenger within the elevator car presses the emergency stop button located normally on the car operation panel.
Typically, the force developed by the brake will be sufficiently large so as to bring an elevator car travelling at 1 m/s to a full halt within 200 ms. This sharp reduction in speed will be uncomfortable and unsettling to any passenger riding in the elevator car and, in some cases, might even lead to injury of the travelling passenger. This problem is understandably further exaggerated in countries throughout the world which experience frequent power disruptions.